1. Field of the Invention
The present invention relates to rock boring machines, and in particular to a new cutterhead construction for such a machine.
2. Description of the Prior Art
U.S. Pat. No. 3,756,332, granted Sept. 4, 1973, to Clayton H. Crane, and assigned to the The Robbins Company, discloses a popular open face cutterhead and a typical cutter arrangement on a cutterhead. The cutterhead includes a plurality of disc cutter assemblies positioned for cutting concentric kerfs in the rock. The disc cutters include sloping breaker surfaces flanking circumferential cutting edges. These breaker surfaces serve to fracture and dislodge the rock material between kerfs which are cut by the cutting edges.
The cutter assemblies are mounted onto the front side of the cutterhead. Most of them are mounted on the front walls of radial spoke beams, while others are mounted on the front side of auxiliary frame structure that is angularly offset from the radial spoke beams. A disadvantage of this type of arrangement is that whenever it is necessary to replace the cutters, which is a frequent occurrence, it is necessary to back the cutterhead up from the tunnel face and then send a workman into the quite dangerous region forwardly of the cutterhead.
It is known to mount the cutters onto the rear side of the cutterhead so that cutter replacement can be made in relative safety behind the cutterhead. However, such rear mounting is most normally associated with closed face cutterheads and has not been used with open face cutterheads. The term "open face tunneling machine" is used herein to mean a cutterhead comprising a plurality of radial spoke beams on which most of the cutters are mounted, with spaces being provided between adjacent beams through which the mined material passes. In a closed face cutterhead the mined material is picked up only at the periphery of the cutterhead, except for pieces which are small enough to pass through the small spaces which exist about each roller cutter element.
Examples of rear mounting of roller cutters onto solid face cutterheads, existing in the patent literature, are disclosed by U.S. Pat. No. 3,358,782, granted Dec. 19, 1967 to Karl G. Bechem, and by U.S. Pat No. 3,444,939, granted May 20, 1969 also to Karl G. Bechem.
For some types of tunnel boring jobs the open face or spoke style of cutterhead possesses advantages over the closed face style of cutterhead in that the cut material can pass through the cutterhead at almost any level of the tunnel face and need not fall down to be picked up by peripheral buckets. A primary object of the present invention is to provide an improved open face type cutterhead on which all of the roller cutters, including a center cutter assembly and gauge cutters, are removable from the back side of the cutterhead.